The present invention concerns a hand or pointer comprising a part made of transparent material and capable of being illuminated so that the hand becomes luminous and visible even in the dark, in particular in an analogue display device such as a timepiece.
The invention also concerns an analogue display device for a timepiece, including such hands. The invention further concerns a watch including such a display device.
Currently, it is common to use a hand made of transparent coloured material which is illuminated internally so that it appears to be luminous, in particular to facilitate night reading of an analogue indicator such as the speedometer of an automobile. One can find for example in Patent publications Nos. WO 96/02810, U.S. Pat. No. 5,546,888 and DE 38 17 874, the description of devices in which a light beam, produced by a fixed source, axially penetrates a transparent rotating hub of the single hand and reaches a reflection surface which distributes the light into the elongated body of the hand. In order for the body of the hand to appear sufficiently and uniformly luminous, incorporating a fluorescent substance therein is known. However, the electric power consumption of these devices has limited their applications to date.
The idea of internally illuminating the hands of a timepiece for the same purpose is already an old one, but cannot be put into practice in the same way as in the aforementioned devices, particularly because of the coaxial arrangement of the hour, minute and possibly second hands, and the superposition of the circular heads of the hands. In order to reach the head of the top hand, the axial light beam has to pass through the head of each underlying hand, where the light flux has to be both divided and directed, on the one hand, towards the body of the hand, and on the other hand towards the overlying hand. Further, since the zone of the rotational axis is generally occupied by an opaque shaft, the light cannot pass into this zone, unless a transparent shaft is used. This thus results in particular difficulties for distributing the light.
In Swiss Patent No. 160 797 published in 1933, each of the two hands of a large clock is made in the form of a hollow housing with a light coloured or transparent bottom and a matt translucent cover. A group of three fixed lamps distributed around coaxial shafts for the hands produces an axial light beam which penetrates the head of each hand from the back, where a mirror distributes light towards the assembly of the bottom of the housing. The mirror of one of the hands is shifted in a radial direction with respect to the mirror of the other to avoid putting it in shadow. Thus, most of the light emitted by the lamps is lost because it passes to the side of the mirrors. Moreover, a construction of this kind with hollow hands is evidently too voluminous to be able to be applied to a watch.
Moreover, it has been proposed to illuminate the hands of a timepiece by means of electro-luminescent elements fixed to the transparent or translucent body of each hand (see for example U.S. Pat. No. 5,623,456). This system has the drawback of requiring rotating electric contacts which take a lot of space and are unreliable in the long term.
In the drawings of Japanese Patent No. 55-60894, one can see a timepiece with two coaxial luminous hands made of transparent material and having light-diffusing surfaces. The superposed circular heads of these hands are illuminated by an axial beam from a fixed source, opposite which the head of the hour hand has an annular groove with a semi-circular transverse profile. Above this groove, the top face of the head includes an annular raised portion provided with a light-diffusing surface for transmitting light towards a similar annular groove of the head of the minute hand. However, this construction has not had any commercial success, probably because of unsatisfactory light distribution in the hands.